Liquid natural gas qualifies as a desirable alternative fuel for internal combustion engines. A major problem associated with the use of liquid natural gas as a fuel for internal combustion engines is that liquid natural gas is a mixture of about 90 to 95% methane with higher molecular weight hydrocarbons, which are called higher hydrocarbons. The principal higher hydrocarbon is ethane, usually in the range of from about 4% to about 7%.
The hydrocarbons higher than methane create several problems for the utilization of liquid natural gas as a fuel for internal combustion engines. First, the higher hydrocarbons have lower auto ignition temperatures than methane.
______________________________________ Critical Auto Ignition Component Compression Ratio Temperature ______________________________________ Methane 13.0 540.degree. C. Ethane 9.8 515.degree. C. Propane 8.8 450.degree. C. Butane 5.3 405.degree. C. Pentane 3.5 260.degree. C. ______________________________________
The composition of natural gas varies widely dependent on the source, Such variation in composition denies engine manufacturers the opportunity to maximize engine designs. The higher hydrocarbons in the liquid natural gas fuel can cause preignition which can cause knock, hot spots and eventually engine failure.
Many processes have been devised for the cryogenic separation from heavier components in a natural gas stream from methane and for cryogenic refrigeration. Among these are U.S. Pat. Nos. 4,072,485 to Becdelievre, et al.; 4,022,597 to Bacon; 3,929,438 to Harper; 3,808,826 to Harper, et al.; Re. 29,914 to Perret; Re 30,085 to Perret; 3,414,819 to Grunberg, et al.; 3,763,658 to Gaumer, Jr., et al.; 3,581,510 to Hughes; 4,140,504 to Campbell, et al.; 4,157,904 to Campbell, et al.; 4,171,964 to Campbell, et al.; 4,278,457 to Campbell, et al.; 3,932,154 to Coers, et al.; 3,914,949 to Maher, et al. and 4,033,735 to Swenson.
Such prior art processes for separation of heavier components from methane utilize complex heat exchange schemes usually involving fractionation in a distillation column. They also start with a natural gas feed stream in the vapor state. Exemplary of such processes is U.S. Pat. No. 4,738,699 to Apffel. The Apffel patent discloses a method for use of a mixed refrigerant refrigeration stream for removing higher hydrocarbons from methane of a natural gas stream. The mixed refrigerant refrigeration system is used to facilitate separation of methane and lighter constituents of the natural gas stream from the higher hydrocarbon components, such as ethane, propane and heavier hydrocarbons. The separation process is accomplished with a fractionation tower, where the methane and lighter gases are separated from the other hydrocarbons using indirect heat exchange with a mixed refrigerant, and a slip stream from the initial feed stream, alternately to provide the energy for distillation.
It is a principle object of the present invention to provide a simple means for providing a purified liquid methane product suitable for use in internal combustion engines from a liquid natural gas source utilizing the liquid natural gas source as the principal refrigerant for the purification and the liquefication of the natural gas.